Aromatics, particularly benzene, toluene, ethylbenzene, and the xylenes (ortho, meta, and para isomers), which are commonly referred to as “BTEX” or more simply “BTX,” are extremely useful chemicals in the petrochemical industry. They represent the building blocks for materials such as polystyrene, styrene-butadiene rubber, polyethylene terephthalate, polyester, phthalic anhydride, solvents, polyurethane, benzoic acid, and numerous other components. Conventionally, BTEX is obtained for the petrochemical industry by separation and processing of fossil-fuel petroleum fractions, for example, in catalytic reforming or cracking refinery process units. The different aromatic compounds can be separated from each in an aromatic complex which has various separation units, as well as processing units for increasing the recovery of specific compounds.
Specifically, para-xylene and meta-xylene are important raw materials in the chemical and fiber industries. Terephthalic acid derived from para-xylene is used to produce polyester fabrics and other articles which are in wide use today. One process which is utilized to produce para-xylene is a toluene methylation process.
The toluene methylation process alkylates toluene using methanol to produce para-xylene at very high (90+%) selectivity relative to total xylenes. Fractionation of the reaction effluent is ultimately necessary to separate the components which include products of light ends, water, oxygenates and acids, benzene, unconverted toluene, xylenes, and heavier components. The separation may be achieved by first performing a “loose” split using a first fractionation column operated for light ends removal and crude recovery of para-xylene. The resulting product stream may then be routed to a second fractionation column, within a larger aromatics complex, for further separation of toluene from the desired xylene products. Alternatively, the xylene separation can be accomplished solely via a fractionation column, wherein practically all of the benzene and toluene are recycled internally so that the only terminal products are xylenes and heavier aromatics.
A disadvantage of the configuration with two columns, and the first one performing a “loose” split, is that oxygenates produced in the toluene methylation reaction that co-boil with toluene are given more opportunity to migrate and damage downstream units in the aromatics complex. For example, oxygenates may damage adsorbent in a para-xylene separation unit. This issue is especially a concern for para-xylene separation units that utilize toluene as a light-desorbent. One advantage, though, is that the first fractionation column is more amenable to heat integration within the aromatics complex versus the second fractionation column, due to its lower bottoms temperature.
However, either configuration necessitates substantially increased fractionation requirements (i.e., condenser and reboiler duties) in the fractionation columns to achieve desired or acceptable separation of toluene and xylene.
Therefore, there remains a need for an effective and efficient process for separation xylene from toluene. Additionally, it would be desired for such processes to reduce the risk of oxygenates migrating downstream.